Polyester resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), which are thermoplastic resins excellent in heat resistance and mechanical properties, have been used in a very wide variety of fields such as plastic films, electronics, energy, packaging materials, and automobiles. Among plastic films, biaxially stretched PET films have been used widely for industrial and packaging fields because of excellent balance between cost and mechanical strength, heat resistance, dimensional stability, chemical resistance, optical properties, etc.
In the field of industrial films, PET films can be used as functional films for flat panel displays (FPD) such as liquid crystal displays and plasma display because of having excellent transparency. Further, PET films provided with hydrolysis resistance have been used for films for solar cell back sheets and also used for various purposes such as functional films and base films.
In the field of packaging films, PET films have been used for applications such as wrapping of foodstuff, shrink labels for bottles, and gas barrier films. Particularly, films excellent in gas barrier properties have been used as packaging materials or gas shielding materials which are required to have air-tightness for foodstuff, pharmaceutical products, electronic parts, and the like, and there has been a growing demand for such films in recent years.
On the other hand, resins having biodegradability and resins produced from biomassderived raw materials have drawn attention as environmentally friendly-type and environmentally sustainable-type materials. From the above-mentioned viewpoint, many investigations have been performed for the purpose of providing a reproducible polymer for replacing petroleum derivatives such as PET. Furandicarboxylic acids (FDCA) have been proposed as compounds having similar chemical natures such as solubility in hot water and stability toward acidic reagents with terephthalic acid which is the main chain skeleton of PET and a planar structure. So it has been proposed to provide furan-based materials by polycondensation of FDCA and a diol (Patent Literature 1 and Non-Patent Literature 1).
Only the melting point in the physical properties of these polymers disclosed is made clear, and mechanical strength is unclear. It is unknown whether or not thermoplastic resin compositions containing a furandicarboxylate unit can be used in the fields of industrial and packaging films.
Polymer compounds usable for electric and electronic parts or the like by specifying the degree of polymerization have been proposed for thermoplastic resin compositions containing, mainly polybutylene furandicarboxylate (PBF), some kinds of furandicarboxylate units (Patent Literature 2). Further, polyesters excellent in mechanical strength by specifying reduced viscosity and terminal acid value have been proposed (Patent Literatures 3 and 4).
However, thermally press-molded products of PBF disclosed in Patent Literature 2 have low transparency and are thus limited for uses in the fields of industrial and packaging films. In the mechanical properties of 200 micro meters sheet products having a polyetylene furandicarboxylate (PEF) structure disclosed in Patent Literatures 3 and 4, both breaking elongation and breaking strength are low and it is not conceivable to use such sheet products in the fields of industrial and packaging films.
Sheets obtained from PEF, PEF derivatives and blends of PEF derivatives and copolymer polyesters have been investigated to be formed into uniaxially stretched films (Patent Literatures 5 and 6).
Patent Literature 5 discloses that as compared with a sheet made of a thermoplastic resin composition containing a furandicarboxylate unit, a film obtained by uniaxially stretching the sheet at 5 to 16 times is improved in breaking elongation, depending on the kinds of blends and the blending ratio of furandicarboxylate unit. However, no significant improvement in breaking elongation is confirmed unless cyclohexanedimethanol-copolymerized PET, which is widely known for improving breaking elongation, is blended. It must be said that the effect is limited depending on the blending ratio and the film has not been used so far in the fields of industrial and packaging films.
Patent Literature 6 discloses a PEF film uniaxially stretched about 1.6 times by using rolling rolls. The film is shown to be a plastic film excellent in gas barrier properties, but it merely mentions the advantages of barrier properties derived from chemical structure of PEF, and mechanical strength which is important for packaging materials is not made clear, so that the film has not been used so far in the field of gas barrier film containing a furandicarboxylate unit for packaging.